The potion is in an unlocked chest in the keep, but taking items from it is a criminal act. Duck and wait until the patrolling guard has fully depleted his "awareness" meter of you before you take the item.

If you are looking for the item, tracking the quest will reveal it; barring that, it is the unlocked chest in the northeast spur of the keep. Get the potion back to Oleander to complete the mission.

Another way to get the potion is to make it yourself. The ingredients for it are one Sky Blossom and one Embereyes. You can find these reagents anywhere around the quest giver, but if you collect early on you should have them in your inventory by now.

You could also just buy it from an alchemist vendor. Nanne in Gorhart should be able to sell you one. The potion is called Minor Alchemist Art.